In the case of conventional memory devices, in particular conventional semiconductor memory devices, one differentiates between so-called functional memory devices (e.g. PLAs, PALs, etc.) and so-called table memory devices, e.g. ROM devices (ROM=Read Only Memory)—in particular PROMs, EPROMs, EEPROMs, flash memories, etc.—, and RAM devices (RAM=Random Access Memory or read-write memory), e.g. DRAMs (Dynamic Random Access Memory or dynamic read-write-memory) and SRAMs (Static Random Access Memory or static read-write-memory).
A RAM device is a memory for storing data under a predetermined address and for reading out the data under this address later. Since it is intended to accommodate as many memory cells as possible in a RAM device, one has been trying to realize same as simple as possible and to scale them as small as possible.
In the case of SRAMs, the individual memory cells consist e.g. of few, for instance 6, transistors, and in the case of so-called DRAMs in general only of one single, correspondingly controlled capacitive element, e.g. a trench capacitor with the capacitance of which one bit each can be stored as charge. This charge, however, remains for a short time only. Therefore, a so-called “refresh” must be performed regularly, e.g. approximately every 64 ms.
In contrast to that, no “refresh” has to be performed in the case of SRAMS since the data stored in the memory cell remain stored as long as an appropriate supply voltage is fed to the SRAM.
In the case of non-volatile memory devices (NVMs), e.g. EPROMs, EEPROMs, and flash memories, the stored data remain, however, stored even when the supply voltage is switched off.
Furthermore, so-called resistive or resistively switching memory devices have also become known recently, e.g. so-called Phase Change Memories and PMC memories (PMC=Programmable Metallization Cell), which are also referred to as CBRAM memories (CB=Conductive Bridging).
In the case of resistive or resistively switching memory devices, an “active” material—which is, for instance, positioned between two appropriate electrodes (i.e. an anode and a cathode)—is placed, by appropriate switching processes, i.e. by appropriate current or voltage pulses of particular intensity and duration, in a more or less conductive state. The more conductive state corresponds e.g. to a stored, logic “One”, and the less conductive state to a stored, logic “Zero”, or vice versa.
In the case of Phase Change Memories (PC memories), for instance, a chalcogenide compound may be used as an active material that is positioned between two electrodes. Chalcogenide compounds are e.g. a Ge—Sb—Te or an Ag—In—Sb—Te compound. The chalcogenide compound material has the property to be adapted to be placed in an amorphous, relatively weakly conductive, or a crystalline, relatively strongly conductive state by appropriate switching processes. The relatively strongly conductive state may, for instance, correspond to a stored, logic “One”, and the relatively weakly conductive state may correspond to a stored, logic “Zero”, or vice versa.
Phase Change Memory Cells are, for instance, known from G.
Wicker, Nonvolatile, High Density, High Performance Phase Change Memory, SPIE Conference on Electronics and Structures for MEMS, Vol. 3891, Queensland, 2, 1999, and e.g. from Y. N. Hwang et al., Completely CMOS Compatible Phase Change Non-volatile RAM Using NMOS Cell Transistors, IEEE Proceedings of the Nonvolatile Semiconductor Memory Workshop, Monterey, 91, 2003, S. Lai et al., OUM-a 180 nm nonvolatile memory cell element technology for stand alone and embedded applications, IEDM 2001, etc.
In the case of PMC memories (PMC=Programmable Metallization Cell)—depending on whether a logic “One” or a logic “Zero” is to be written into the cell—conductive bridges (e.g. of Ag or Cu, etc.) are built up during the programming of a corresponding PMC memory cell by means of current or voltage pulses of particular duration and intensity, and by electrochemical reactions caused thereby, in an active material positioned between two electrodes, which results in a conductive state of the cell, or are broken down again, which results in a non-conductive state of the cell.
PMC memory cells or CBRAM memory cells, respectively, are e.g. known from Y. Hirose, H. Hirose, J. Appl. Phys. 47, 2767 (1975), and e.g. from M. N. Kozicki, M. Yun, L. Hilt, A. Singh, Electrochemical Society Proc., Vol. 99-13, (1999) 298, M. N. Kozicki, M. Yun, S. J. Yang, J. P. Aberouette, J. P. Bird, Superlattices and Microstructures, Vol. 27, No. 5/6 (2000) 485-488, and e.g. from M. N. Kozicki, M. Mitkova, J. Zhu, M. Park, C. Gopalan, “Can Solid State Electrochemistry Eliminate the Memory Scaling Quandry”, Proc. VLSI (2002) and R. Neale: “Micron to look again at non-volatile amorphous memory”, Electronic Engineering Design (2002).
CBRAM memories are, for instance, described in Y. Hirose, H. Hirose, J. Appl. Phys. 47, 2767 (1975), T. Kawaguchi et al., “Optical, electrical and structural properties of amorphous Ag—Ge—S and Ag—Ge—Se films and comparison of photo-induced and thermally induced phenomena of both systems”, J. Appl. Phys. 79 (12), 9096, 1996, and e.g. in M. Kawasaki et al., “Ionic conductivity of Agx(GeSe3)1−x (0<x0.571) glasses”, Solid State Ionics 123, 259, 1999, etc.
In the case of CBRAM memory cells, an electro-chemically active material is positioned in a volume between two electrodes, for instance, an appropriate chalcogenide material e.g. in a GeSe, GeS, AgSe, or CuS compound. In the case of the CBRAM memory cell, the above-mentioned switching process is based on the fact that, by applying appropriate current or voltage pulses of particular intensity and duration to the electrodes, elements of a so-called deposition cluster increase in volume in the active material positioned between the electrodes until the two electrodes are finally bridged electroconductively, i.e. are electroconductively connected with each other, which corresponds to the electroconductive state of the CBRAM cell.
By applying correspondingly inverse current or voltage pulses, this process can be reversed again, and the corresponding CBRAM cell can be placed in a non-conductive state again. This way, a “switching” between a state with a higher electroconductivity of the CBRAM memory cell and a state with a lower electroconductivity of the CBRAM memory cell can be achieved.
The switching process in the CBRAM memory cell is substantially based on the modulation of the chemical composition and the local nanostructure of a chalcogenide material doped with a metal, serving as solid body electrolyte and diffusion matrix. The pure chalcogenide material typically has a semiconductor behavior and has a very high electric resistance at room temperature, the resistance being by magnitudes, i.e. decimal powers of the ohmic resistance value, higher than that of an electroconductive material. By the current or voltage pulses applied via the electrodes, the steric arrangement and the local concentration of the ionically and metallically available components of the element mobile in the diffusion matrix is changed. By that, the so-called bridging, i.e. an electric bridging of the volume between the electrodes of metal-rich depositions, can be caused, which changes the electric resistance of the CBRAM cell by several magnitudes by the ohmic resistance value being decreased by several decimal powers.
One difficulty with this switching process in a CBRAM memory cell consists in that the electric resistance between the electrodes may vary relatively strongly with a particular state of the cell (“conductive” or “non-conductive”). This variation aggravates the evaluation or the differentiation, respectively, between the conductive and the non-conductive state by a corresponding evaluation circuit. This means that it is aggravated to determine whether a logic “Zero” or a logic “One” was last stored in the corresponding memory cell. A further difficulty consists in that the CBRAM memory cell does not comprise any reproducible switching properties in the “unconditioned” state. It is therefore necessary to condition the memory cell, i.e. to be able to exactly control the doping of the memory cell, to achieve a reproducible switching behavior.
The CBRAM memory cell therefore has to be conditioned prior to the above-described switching behavior. This means that the doping of the chalcogenide matrix positioned between the electrodes has to be adjusted reproducibly by a mobile, metallic element so as to achieve a good control of the overall concentration or a good controllability of the metal element, respectively, and thus a good control of the electric resistance in the CBRAM memory cell.
The conditioning of a CBRAM memory cell has so far been performed e.g. by means of photo diffusion, i.e. a probe that may, for instance, be generated by a metal layer on a chalcogenide material is exposed with light in the ultraviolet frequency range, this causing the metal to be driven into the probe. In literature, this method is also referred to as photo diffusion and results in a particular metallic doping profile of the chalcogenide material in which metal-rich depositions form in the chalcogenide matrix. This way, a doped and an undoped phase will be generated in the chalcogenide layer.
Other methods by means of which a conditioning of the probe is achieved are, for instance, thermal methods where a regular diffusion occurs, or implantation methods. These methods may also be combined to obtain a doping of the chalcogenide matrix.
A drawback of the thermal method consists in that the amorphicity of the probe may be lost since a nanocrystallization with subsequent grain growth occurs which substantially changes the nanostructure or microstructure, respectively, of the probe. The ion movability of the metal doping substance is, however, as a rule by magnitudes smaller in crystalline materials, which may involve a significant degradation of the memory cell properties.
A drawback of the photo diffusion process consists i.a. in that the doping profile is very steep due to the photo-stimulated doping process since the movability of the ions is distinctly higher in the metal-rich phase. This results in an extremely critical process control since, as soon as the steep doping edge of the photo diffusion profile at the limiting area has reached the opposite electrode, the memory cell is irreversibly electrically short-circuited. If, however, the photo diffusion profile does not expand far enough through the chalcogenide matrix, an electric forming pulse is additionally required which drives the metallic material thermally into the chalcogenide material by means of local heating. The electric forming pulse is, however, incompatible with some semiconductor manufacturing processes of mass products since an electrical conditioning does not guarantee sufficient reproducibility.
A drawback of the implantation process consists in that extremely high doses of metal have to be implanted, which requires a very high implantation performance and/or a very long duration of the implantation process. A further difficulty results from the fact that the implanted doping courses are formed very flatly since otherwise an undesired mixing or, in the case of too deep implantation profile courses, an electric short-circuit will be caused.